Coatings of Polymers and Plastics Part 10 potx

214 Schoff F IG .9 Solvent popping. (From Ref. 5, used with permission.) pores in the plastic, although other gases may be involved. To reduce or prevent this defect, it is necessary to use a primer that seals the SMC surface well. Gassing or blowout is possible over other plastics as well. Any product that has bubbles and pores, especially close to the surface, has a potential for this defect. Another volatile-related defect, air entrapment, is a problem for many coatings. Agitation during manufacture, handling, or application may cause air to mix in or dissolve in the paint. On application, the air tries to leave the film, but often is trapped or the bubbles break late in the film formation process so that holes are left that do not flow out. The result often is difficult to distinguish from solvent pops or gassing. A defect that I have seen a number of times on painted plastics is what I call micropopping (Fig. 12). Very small (0.4–2 mils or 10–50 µm in diameter) bubbles, bumps, or pinholes appear in the film, often late in the bake. To the eye, the result may look like haze, give fuzzy reflections or just an appearance that does not look right. The cause may be trapped solvent, volatile by-products of the curing process, or even clumps of flow-control agent or pigment. Micropopping coupled with bumpiness that often occurs on shrinkage during cure can turn a smooth glossy surface (when wet or early in the bake) into a rough ugly one. Painting Problems 215 F IG .10 Gassing from a plastic substrate. Note the hole in the center of the defect. 3.4 Flow-Related Defects A number of the defects described previously involve flow driven by surface tension, but there also are flow defects where surface tension has little or no involvement. When a paint is applied, it is expected to flow out and level to produce a smooth film. Unfortunately, this does not always happen. Sometimes the viscosity is so high when the paint arrives on the part or increases so quickly after application that there is little flow and the result is a rough, bumpy surface F IG .11 Diagram of the cross section of a gassing defect. (From Ref. 5, used with permission.) 216 Schoff F IG .12 Example of micropopping. Perhaps, microbumps is a more accurate de- scription in this case. To the eye, the effect is one of fuzziness of reflections. like the peel of an orange (Fig.13). In base/clear systems, the clear usually is blamed for orange peel, but a basecoat with poor flow can cause this as well. This usually is due to telegraphing of the basecoat topography so that the clear really is bumpy, but I have seen cases in which there was an optical illusion where clear was smooth, but the rough basecoat showed through. If the paint viscosity is too low after application or too much is applied, the paint may flow too well on vertical surfaces (particularly at holes or style lines), causing ugly sags, runs, and slumps. Even if thicker areas are not considered objectionable to the eye, popping may occur in them. 3.5 Other Defects 3.5.1 Dirt The most common defect of all is dirt. This defect rarely is a concern to formu- lators, yet is a serious problem in most plants where their paints are applied. Most dirt comes from the paint user’s facility. Occasionally, the dirt is in the paint as it arrives in the plant and sometimes pigment flocs or seeds form during Painting Problems 217 F IG .13 Orange peel as seen through the microscope. the circulation of the paint, but both of these are rare compared to dirt from the plant and the people who work in it. There are many different kinds of dirt such as fibers (Fig. 14), sanding dust, resin gel particles, dried paint particles and chips, oven dirt (Fig. 15), rust, etc. Dirt sources include clothing; wiping cloths; tack rags; gloves; faulty or clogged filter bags that break; overhead chains and carriers; racks and hangers; ovens; compressed air for application; food (eating in the booth); dried paint in pipes and hoses; roof leaks; rust and flaking paint in booths and tunnels; hoses that are disintegrating, etc. Road and construction dust, truck and locomotive exhaust, pollen, insects, fly ash, soot, and other particulates may enter from outside the plant. Sometimes plant exhaust and inlet pipes are positioned so that plant exhaust is pulled back into the paint shop or paint area when the wind blows in a certain direction. Poor work practices such as playing around on the paint line, the wearing of nonapproved gloves and clothing, sprayers not wear- ing gloves, use of booths as passageways, and careless tacking and wiping all can cause dirt or make it worse. Some defects that look like dirt really are due to application problems. Examples are spits, drops, and overspray. Worn, dam- 218 Schoff F IG .14 Dirt—an example of a fiber. aged, or dirty application equipment; too much shaping or fan air; excessive paint flow rates; excessive voltage; and loose or overtightened caps and nozzles all can cause “dirt.” 3.5.2 Color Problems Color is a surface attribute, but a coating being the wrong shade is not a surface defect in the usual sense. Color matching of coated parts made of different plastics or of metal and plastic can be very difficult. In an auto plant, the plastic always is expected to match the painted steel, but I have been on lines where it turned out that the original equipment manufacturer (OEM) coating was the wrong color, not the one on the plastic parts. Color can be affected by film thickness, by the method of application, whether there is pigment flocculation or not, etc. A batch of paint may be the correct color to begin with, but on circulation, the pigment may slowly flocculate causing the color to drift. A similar thing can happen with aluminum flake in a metallic paint. The parts become darker with paint circulation time as the flake clumps together and no longer reflects light. Sometimes an off color is caused by a colored impurity, usually yellow or pink. This may be in the paint or reducing solvent or may Painting Problems 219 F IG .15 An example of oven dirt. migrate up from the plastic or another coating layer. Amines and various addi- tives, including UV absorbers, have been blamed for such problems. 4 FIELD PROBLEMS/FAILURES Field failures may seem to be completely different from painting problems, but they may be connected to a greater extent than we realize. An excellent source of information in this area is Ref. 20. The author points out that for a coating to fail, it must be stressed. How it responds to this stress depends on the physical and mechanical properties of the coating and its interface with the substrate. These, in turn, depend on the chemistry of the coating and the degree of cure, but may also be affected by the application process, defects in the coating, or repairs to defects. Let us consider some field failures. 4.1 Adhesion The most serious problem in paint for plastics is loss of adhesion to the plastic. Paint is useless if it does not stick. Adhesion failure may occur soon after appli- 220 Schoff cation or may occur later in the field. The failure may involve very small areas or very large ones. A high stress such as scraping may be necessary or the paint may sheet off seemingly with little or no force. As far as a paint user is con- cerned, a failure is a failure regardless of the magnitude, timing, or force that is needed. However, these differences are very significant to someone who solves problems. For example, failure after some period of time or failure with little stress may mean that a plastic component or additive has migrated to the plastic- coating interface giving an intermediate layer. This weak boundary layer will interfere with the plastic-coating bond, yet will have little or no cohesive strength of its own, so adhesion failure occurs. What does it take to achieve good adhesion? The first requirement is inti- mate contact between two surfaces. This is where wetting comes in. However, wetting is a necessary, but not sufficient condition for good adhesion of a dried or baked coating. In fact, there are paints that wet surfaces very well, but are designed to be temporary and that can be peeled off easily once they are baked and cured. Wetting does involve adhesion of the liquid paint to the substrate, but loss of solvent and other changes may destroy this bonding. The second requirement is that one material must adsorb on the other. In order to do this, they must be highly compatible with each other. There is an old adage that “like dissolves like.” We also can say that “like wets like” and “like adheres to like.” The third requirement is that there be polar groups on both materials to aid in the formation of adhesive bonds. There is evidence in the literature (21–23) that matching the polarities of the cured paint and the substrate contributes to good adhesion. This explains why polar coatings do not stick very well to nonpolar or low polarity plastics. Adhesion promoters are based on the concept of linking unlike materials by having a two-faced layer that shows one face to the nonpolar plastic and a very different face to the polar paint. This works even better if the promoter solvents swell the plastic and allow penetration by the polymer chains in the promoter. Adhesion failures over plastics sometimes only occur in certain places on parts, such as on the corners of bumpers or in the area of the mold gate. Analysis has shown that these areas have different compositions or different degrees of crystallinity from the rest of the surface of the part. This can be due to tempera- ture differences in the mold, imperfect mixing, or different stresses and strains during filling and cure. One possible cause of adhesion loss is degradation of the surface of one of the intermediate layers (primer, adhesion promoter) or the plastic by ultravio- let light. It only takes a small amount of degradation at an interface to hurt adhesion. The topcoats are supposed to protect the layers below, but thin clear- or basecoat films, low pigment loadings, and loss of UV absorbers can allow UV transmission. Pigments provide UV protection by blocking out the light and Painting Problems 221 many also absorb UV. Additives are used to absorb UV light and change the energy to heat energy or act as free radical scavengers. 4.2 Mar and Scratch The terms mar and scratch refer to surface damage due to contact with sharp or rough objects. There is general agreement that a scratch is a mark or injury produced on a surface by something that is sharp or has a ragged edge. It often involves fracture of the surface. Unfortunately, the term mar is not well defined and means different things to different people. It may be used to refer to any surface damage or only to certain kinds such as abraded or off-color spots or areas. Damage can occur in many places. Painted parts are exposed to a number of possible scratch and abrasion situations even before they become part of a car, piece of equipment, or other object. Handling, packaging, storage, and ship- ping all are operations that can result in damage. This is compounded by the fact that many coatings take time to develop resistance and may be easily scratched immediately after painting. During use there are many additional dangers for the surface. Fortunately, unless they are undercured, coatings for plastics tend to be tough and flexible and most have reasonably good scratch resistance after their initial tenderness. The surface often deforms instead of fracturing and the resultant indentation or groove can heal, especially in warm weather. 4.3 Friction-Induced Damage (Gouging)—Bump and Rub Impacts A type of defect that occurs on painted thermoplastic olefin (TPO), particularly on auto bumpers is damage that occurs when the bumper rubs against a post, wall, or other immovable object. A strip of coating shears off along with the top layer of the TPO. Resistance to such damage does not seem to be related to the adhesion between the coating and the substrate, but rather to the cohesion between the surface layer and bulk of the TPO. 4.4 Stone Chipping Cars and trucks are continually bombarded by stones thrown up by their own tires and those of vehicles in front of them or passing them. Many parts of North America have gravel roads and/or gravel shoulders on paved roads. Even paved roads can have loose material. Some vehicle designs (sloping hoods, tires that stick out beyond fenders, lack of mudguards or protective coverings at the back of fender cutouts, etc.) invite damage. Considering all of this, stone chipping is a surprisingly minor problem and coated plastics suffer far less than coated metal. Resistance to stone chipping depends on having a combination of excellent adhe- 222 Schoff sion of all layers, good mechanical properties, and the ability to absorb much of the energy of the stone as it strikes the surface. Plastics tend to give with the impact, whereas metals do not. Damage is possible, but warranty claims and cus- tomer complaints are rare, so there is less concern than with other defects. 5 TOOLS A worker needs tools and so does a coatings problem solver. By tools, I mean principles and techniques as well as instruments. Many tools are available and I will discuss a number of them. 5.1 General Tools 5.1.1 Light Microscope This is the most useful single piece of equipment for solving defect problems. It is good to use two of them, a low power (5–100X) stereo microscope for looking at defects and a higher power (100–1000X) one for cross sections, examining wet paint, etc. Microscopes should be connected to still or video cameras for documentation of what is seen. Video cameras can be used to print still pictures (using a videoprinter) or videotape the baking process and the formation of defects. Addition of a capture board and image analysis software enables the investigator to take and store pictures, insert them into documents, send them by email, etc. 5.1.2 Root Cause Analysis Methodology Root cause analysis involves the determination of the basic or underlying cause of a defect or problem and the providing of evidence that it is the cause. We know that craters are caused by contaminants, but the root cause of a cratering outbreak may be poor tote cleaning, a contaminated drum, overreduction of the paint so that it flows too much, or a batch of paint that is unusually sensitive to contaminants that always are present. It may be clear that a defect is a solvent pop, but the root cause could be an application problem that causes fat edges or sags that, in turn, lead to pops. Root cause analysis often takes a lot of detective work, experimentation, and documentation. Sometimes it takes longer than it did to solve the problem. The point is that if the true root cause has been identi- fied and removed or fixed, the problem or defect should not occur again. 5.1.3 Regular Audits Audits for dirt, craters, to measure whether improvements have occurred, whether best practices are being followed, condition of application equipment, whether there is oil in the compressed air, etc. are very important for reducing and ultimately preventing painting problems. Such audits should be done on a Painting Problems 223 regular basis and ratings should be done against standards. Audits can and should be incorporated into ISO 9000 or other quality process methodology. Self-auditing by teams or departments is important and useful, but exchange audits by people from other plants or parts of an organization also should be done. 5.2 Tools for Characterizing Wetting and Wettability 5.2.1 Wetting Tests The main technique for investigating wetting problems is the measurement of the contact angle of a specific liquid or liquids on the surface of interest. This normally is an advancing angle, that is, during formation the drop advances across the surface. The receding contact angle where a drop retracts over a previously wetted surface would seem to be more useful for characterizing de- wetting phenomena, but it is rarely measured in the coatings industry. Critical Surface Tensions. Much wettability testing owes its basis to Zis- man and his critical surface tension of wetting (24,25). The contact angles of various liquids (often a homologous series of hydrocarbons) on the surface are determined and the contact angles are plotted versus the surface tensions of the liquids (see Fig. 16). The plot is extrapolated to cos θ=1, that is, θ=0°, which represents the point where the liquid would just spontaneously spread if applied as a drop. This point defines the critical surface tension, γ c . As long as the F IG .16 Critical surface tension (Zisman) plot for wettability of polytetrafluoroethy- lene by n-alkanes (25). The parameter γ c is the critical surface tension. (From Ref. 5, used with permission.) [...]... significant differences between test plaques and line parts and in day-to-day or week-to-week results on line parts This is why it is crucial to paint and test actual parts and not totally rely on data from painted plaques The O-W-K method has been applied to a number of lab and field problems and has been found to be very useful in explaining and predicting wetting and adhesion failures (4,5,26) In addition,... However, with metals and coatings, dispersion component values often are close to critical surface tensions, but this does not seem to hold for most of the plastics in this table The polarities of many of the plastic surfaces were higher than I expected This may reflect the effectiveness of treatment and cleaning rather than basic surface properties TABLE 2 Solid Surface Tensions of Polymers by Owens-Wendt-Kaelble... Prevention of dirt problems requires clean raw materials; clean paint; a clean paint shop (isolated from the rest of the plant); a clean, dry air supply (environmental and compressed air); low-lint protective clothing and wiping cloths; and a properly trained, disciplined workforce Sanding should be minimized and vacuum sanders should be used where possible Dirt and dust should be removed by good tacking and. .. Skrovanek The assessment of cure by dynamic mechanical analysis Progr Org Coat 18:89 101 , 1990 9 LW Hill Structure/property relationships of thermoset coatings J Coat Technol 64(808):29–41, 1992 10 D Kranbuehl In-situ cure monitoring of coatings for plastics Proceedings, Div Polym Mater Sci Engr, Amer Chem Soc, 63:90–93, 1990 11 DD Shepard Studying the drying and curing rates of acrylic automotive topcoats... Surface-tension-driven flows of coatings: bondline readout formation J Coatings Technol 73(918):63–71, 2001 19 RE Johnson Jr, RH Dettre Wetting of low energy surfaces In: JC Berg, ed Wettability Surfactant Science Series, vol 49 New York: Marcel Dekker, 1993, pp 1–73 20 DG Weldon The Failure Analysis of Paints and Coatings Chichester, England: John Wiley and Sons, 2000 238 Schoff 21 U Zorll Practical... organic coatings In: GD Parfitt and AV Patsis, eds Proceedings of the 8th International Conference in Organic Coatings Science and Technology, Athens, July 1982, pp 301–328; Organic Coatings Science and Technology, vol 6 New York: Marcel Dekker, 1984, pp 325–350 24 WA Zisman Relation of the equilibrium contact angle to liquid and solid constitution In: RF Gould, ed Contact Angle, Wettability and Adhesion... where γl is the surface tension of the test liquid and γs is the surface tension of the solid in question An equation is written for each liquid All the quantities are known except γ d and γ p, the dispersion and polar components of the SST s s 226 Schoff We are left with two equations in two unknowns and these can be solved to give the unknown values The dispersion and polar components are then added... VM&P naphtha), and peel testing (58,64) Chip, scratch, and abrasion tests can be useful for evaluating adhesion, although mainly to show gross differences 5.6.2 Mar and Scratch There are a large number of mar -and- scratch tests Some are attempts to recreate field damage such as from car washes; polishing and dry wiping; and general use and operation Other tests involve the measurement of basic mechanical... Pierce and CK Schoff Coating Film Defects, rev ed Blue Bell, PA: Federation of Societies for Coatings Technology, 1994 5 CK Schoff Surface defects: diagnosis and cure J Coat Technol 71(888):56–73, 1999 6 LW Hill, K Kozlowski Crosslink density of high solids melamine-formaldehyde cured thermoset coatings J Coat Technol 59(751):63–71, 1987 7 T Provder Cure characterization in product research and development... nonpolar and two polar, having known Lewis acidity and basicity are measured From the results, data such as that shown in Table 5 can be calculated and the presence of acidic and basic sites established (γ + for acidic and γ − for basic) These data indicate that the polymethylmethacrylate (PMMA) surface was basic and that chlorinated polyvinylchloride (CPVC) and polyvinyl flouride (PVF) were both acidic and . layer and bulk of the TPO. 4.4 Stone Chipping Cars and trucks are continually bombarded by stones thrown up by their own tires and those of vehicles in front of them or passing them. Many parts of. undercured, coatings for plastics tend to be tough and flexible and most have reasonably good scratch resistance after their initial tenderness. The surface often deforms instead of fracturing and the resultant. or off-color spots or areas. Damage can occur in many places. Painted parts are exposed to a number of possible scratch and abrasion situations even before they become part of a car, piece of